? Annelids | ||||
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| [image] Leech | ||||
| Scientific classification | ||||
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| Classes and subclasses | ||||
| Class Polychaeta (paraphyletic?) Class Clitellata Oligochaeta - Earthworms and others Acanthobdellida Branchiobdellida Hirudinea - Leeches Class Myzostomida Class Archiannelida (polyphyletic) Class Echiura | ||||
| *Some authors consider the subclasses under Clitellata to be classes |
The annelids, collectively called Annelida (from Latin annellus "little ring"), are a large phylum of animals, comprising the segmented worms, with about 15 000 modern species including the well-known earthworms and leeches. They are found in most wet environments, and include many terrestrial, freshwater, and especially marine species (such as the often gorgeous polychaetes), as well as some which are parasitic or mutualistic. They range in length from under a millimetre to over 3 metres.
Anterior to the true segments lies the prostomium and peristomium, which carries the Mouth, and posterior to them lies the pygidium, where the Anus is located. The digestive tract is usually specialized. Different species of annelids have a wide variety of diets, including active and passive hunters, scavengers, filter feeders, direct deposit feeders which simply ingest the sediments, and blood-suckers.
The vascular system and the Nervous system are separate from the digestive tract. The vascular system includes a dorsal vessel conveying the blood toward the front of the worm, and a ventral longitudinal vessel which conveys the blood in the opposite direction. The two systems are connected by a vascular sinus and by lateral vessels of various kinds, including in the true earthworms, capillaries on the body wall.
The nervous system has a solid, ventral nerve cord from which lateral nerves arise in each segment. Every segment has an autonomy; however, they unite to perform as a single body for functions such as locomotion. Growth in many groups occurs by replication of individual segmental units, in others the number of segments is fixed in early development.
Hermaphrodite annelids like earthworms mate periodically throughout the year in favored environmental conditions. Earthworms mate by Copulation. Two worms which are attracted by each other's secretions lay their bodies together with their heads pointing opposite directions. The fluid is transferred from the male pore to the other worm. Different methods of Sperm tranference have been observed in different genera, and may involve internal spermathecae (sperm storing chambers) or spermatophores that are attached to the outside of the other worm's body.
Most polychaete worms have separate males and females and external fertilization. The earliest larval stage, which is lost in some groups, is a ciliated Trochophore, similar to those found in other phyla. The animal then begins to develop its segments, one after another, until it reaches its adult size. The oligochaetes and leeches tend to be hermaphroditic and lack free-living larvae of this sort. While annelids have some regenerative abilities, sometimes to the point where each half of an adult divided cross-wise will survive, this is not universal, and especially does not occur among the earthworms as folklore would suggest.